Iridium-Catalyzed Enantioselective Hydrogenation of Terminal Alkenes

Iridium complexes derived from chiral P,N ligands are efficient catalysts for the enantioselective hydrogenation of 2‐aryl‐substituted terminal alkenes. Using 0.1–1 mol % of catalyst at room temperature and ambient hydrogen pressure, high enantioselectivities (88–94% ee), full conversions after short reaction times and essentially quantitative yields were obtained for a range of differently substituted 2‐arylalkenes. Among six iridium complexes that were tested, the most selective catalyst was a complex with a phosphinite‐oxazoline ligand derived from threonine (Ir‐ThrePHOX). In contrast to the hydrogenation of trisubstituted alkenes, a strong pressure effect was observed for this class of substrates. Lowering the hydrogen pressure from 50 to 1 bar resulted in a strong increase of the ee values.